We've not made any significant changes to tags yet. It's still
something we're interested in doing, but it's not made it to active
development yet. You cannot currently merge tags, but you can bulk
edit tickets to add tags to multiple tickets.

You can access the bulk edit option on the tickets page (ex.
https://collectorz.lighthouseapp.com/projects/XXX-project/tickets).
Once you're on that page, click on "Bulk Edit Tickets" in the
subnav and then select the tickets you want to edit. Bulk edit is
also available on search results so you can search for something
like tagged:foo and then add/remove tags as you would
want.